.container {
  height: 130px;
}
.box_1 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  border: 1px solid black;
  animation: b_bottom 5s infinite;
}
@keyframes b_bottom {
  50% {
    border-bottom: 15px solid lightcoral;
  }
}